1. Field of the Invention
The present invention relates to an imaging lens having a fixed focal point which is suitable for being incorporated into a digital camera using an imaging element, such as a CCD (Charge-Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor), or a compact imaging device such as a camera using a silver film.
2. Description of Related Art
In step with recent popularization of personal computers into ordinary households, a digital still camera capable of inputting image information, such as a captured landscape or portrait, into a personal computer (hereinafter simply called a “digital camera”) has become quickly widespread. Moreover, there is an increase in the number of cases where a portable cellular phone is equipped with a module camera (a portable module camera) for inputting an image.
An imaging element, such as CCD or CMOS, is used in these imaging devices. With a progress in miniaturization of the imaging elements, there is a demand for miniaturization of an overall imaging device and an imaging element incorporated into the imaging device. Simultaneously, the number of pixels in an imaging element is also on the increase, and higher resolution and performance of the imaging lens are demanded.
Accordingly, JP-A-10-48516, JP-A-2002-221659, JP-A-2004-302057, JP-A-2004-341013, JP-A-2005-24581 and JP-A-2005-24889 describe an imaging lens which is built from three or four lenses and which utilizes aspheric surfaces. For example, in JP-A-2004-341013, JP-A-2005-24581 and JP-A-2005-24889, an attempt is made to achieve further miniaturization and higher performance by means of power arrangement of four lenses, in order from an object side, positive power (positive refractive power), negative power (negative refractive power), positive power, and positive power and using an aspheric surface for each of the lens surfaces. An absolute value of a radius of curvature of an image-side surface of a third lens is reduced, to thus impart comparatively-high positive power to the image-side surface.
As mentioned above, progress has recently been made in miniaturizing an imaging element and increasing the number of pixels thereof. With the progress, higher resolution performance and further miniaturization of a configuration are requested particularly of an imaging lens for use in a digital camera. Although primary demands for an imaging lens for use in a portable module camera have hitherto been cost and miniaturization, there is recently a tendency toward an increase in the number of pixels of an imaging element even in connection with the portable module camera, and a demand for higher performance of the imaging lens is also increasing.
Therefore, development of a wide variety of lenses which are generally enhanced in terms of cost, imaging performance, and compactness is desired. For instance, development of a low-cost high-performance imaging lens which ensures compactness enabling incorporation into a portable module camera with an eye toward incorporation into a digital camera in terms of performance are desired.
In order to meet the demands, an example conceivable measure is to adopt three or four lenses with a view toward miniaturization and cost reduction and to actively use an aspheric surface in order to enhance performance. In this case, an aspheric surface contributes to miniaturization and enhancement of performance but is disadvantageous in terms of ease of manufacture and likely to induce an increase in cost. Hence, it is desirable to taken into sufficient consideration ease of manufacture at the time of use of the aspherical surface. The lenses described in the documents mentioned above adopt a configuration where three or four aspherical surfaces are used. However, in terms of achievement of both imaging performance and compactness, the configuration is insufficient.